The Mobile Backhaul, MBH, segment is the part of a communications network located between the Base Stations, BS, of a wireless communications network, located at cell sites, and the communications network core segment. In traditional radio access network, RAN, architectures, radio and baseband processing are integrated in a BS which typically outputs a Gigabit Ethernet (GbE/10 GbE) signal. GbE clients are transported across the MBH segment using microwave signals and/or over optical fibre. Wavelength division multiplexing, WDM, can be used to increase the aggregated capacity of the optical fibre, especially in situations when fibre availability is scarce or limited by topology or cost constraints.
In the Coordinated RAN, C-RAN, architecture, radio and baseband processing are separated. Remote Radio Units, RRU, which include radio, amplification/filtering, and digital-to-analogue, D/A, conversion, are located at the cell site. Digital Units, DU, for performing baseband processing, are separated from the RRUs, and are sometimes aggregated to form a DU pool. Decentralizing the DU enables improved coordination of radio capabilities across a set of RRUs, faster service delivery, and cost savings. Common Public Radio Interface, CPRI, as defined in the CPRI Specification, is the radio interface protocol widely used for IQ data transmission between RRUs and DUs.
Both RAN and C-RAN architectures can benefit from using a transport layer based on optical technologies, which ensures low propagation delays, high data throughput, and low power consumption, while being an economical choice in exploiting fibre resources. In RAN, GbE clients can be transported over optical channels, directly mapped over wavelengths or as Optical Transport Network, OTN, clients as defined in ITU-T Recommendation G.709. In C-RAN, CPRI flows can also be transported over optical channels. ITU-T has recently included CPRI as a further OTN client in an appendix to G.709. However, the actual implementation of CPRI over OTN still presents unsolved issues. A simpler alternative to OTN has been proposed in PCT/EP2014/052056 in which CPRI is mapped over WDM optical channels with no or only a small additional overhead for Operations, Administration and Maintenance, OAM, and forward error correction, FEC, purposes. This may be referred to as CPRI over WDM.
A third option is CPRI over Ethernet, CoE, which replaces a direct dedicated link between DU and RRU, using the CPRI protocol, with an Ethernet link having a CoE adapter at both the RRU and DU. There are two primary functions of a CoE adapter: implementation of the precision CoE clock; and mapping ingress CPRI basic frames received from the RRU or DU into outgoing CoE packets. The inverse mapping is applied to incoming CoE packets. CoE is appealing since it allows, in principle, management of any kind of traffic in the same Ethernet switch. However, due to the nature of Ethernet, CoE packets arrive asynchronously and some of them may be missing. Hence, a CoE Packet Buffer is needed to assemble arriving CoE packets in the order of egress from the originator. While the CoE packets arrive asynchronously at the adapter over the Ethernet interface, they must be delivered into the CPRI interface synchronously in order to be transparent to the CPRI interface. This is accomplished by transferring CPRI basic frames extracted from the CoE packets into the CPRI interface at both a precise known latency and at a clocking rate that is regulated by the local CoE clock.